Automotive exhaust component and method of manufacture

ABSTRACT

A catalytic converter for an internal combustion engine exhaust system comprises a single-piece, seamless metal housing having tubulated gas inlet and outlet ports and a tubulated intermediate section with a plurality of cascaded catalytic element therein. The intermediate section is connected to the inlet port by an inlet transition section and to the outlet port by an outlet transition section. The inlet and outlet ports and the inlet and outlet transition sections are formed by swaging the ends of a seamless tube used to form the housing. Exhaust gas produced by operation of the engine passes into the converter and through the catalytic element. Noxious substances in the exhaust, including CO, NO x , and incompletely combusted hydrocarbons are converted to more benign substances through the action of the catalytic element, which is preferably a frangible ceramic honeycomb structure having a plurality of internal passages coated with a catalytically active substance. A swaging process is used to form tubulated ends on the converter. The tabulated ends minimize production of turbulence in the gas flow and allow the converter to be connected to the rest of the exhaust system by clamped, welded, or flanged joints. The one-piece, seamless construction of the converter is economical to produce and eliminates welding of housing components that tend to fail when subjected to corrosive exhaust gasses over a prolonged period of time.

[0001] This application is a continuation-in-part of co-pending U.S.patent application Ser. No. 10/389,868, filed Mar. 18, 2003, and furtherclaims the benefit of provisional U.S. Patent Application Ser. No.60/367,419, filed Mar. 26, 2002. Each of application Ser. Nos.10/389,868 and 60/367,419 is incorporated herein in its entirety byreference thereto.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to the field of automotive exhaustcomponents; and more particularly, to a muffler, catalytic converter orthe like, that is formed and housed within a seamless enclosure.

[0004] 2. Description of the Prior Art

[0005] It is widely recognized that the exhaust emissions of internalcombustion engines constitute a major source of air pollution throughoutthe world. The combustion process in these engines inevitably results inthe production of certain substances that pass into the exhaust streamand are detrimental to the health and well being of humans and otheranimal and plant species. The emissions of concern include particulates(soot) along with gases such as CO, SO₂, NO_(x), and imperfectly burnedhydrocarbons (HC). These substances are produced in the combustionprocess, along with the CO₂ and H₂O that are the products of thecomplete oxidation of the hydrocarbons comprised in fuel.

[0006] The combination of market forces and governmental environmentalregulations has spurred research and development of ways to mitigate oreliminate the production of the harmful constituents in engine exhaust.Automakers and suppliers have been challenged to control and reducevehicle tailpipe emissions by the U.S. Clean Air Act of 1965 andsubsequent legislation in the U.S. and other countries. In response tothis legislation, virtually every system in the engine has beenimproved. As a result, modern engines more efficiently convert thelatent chemical energy in fuels to useful mechanical work, so that theiremissions are markedly reduced.

[0007] To date significant efforts have been directed toward thefour-stroke Otto engine in passenger automobiles, owing to consumerpreferences and government action. Despite progress in emissionreduction for these automobile engines, increasingly stringent limitsare being imposed. Emission regulations have also extended to otheron-road vehicles, such as busses, and trucks, many of which employdiesel engines; to off-road vehicles; and to non-propulsion engines,many of which are two-stroke.

[0008] Much of the reduction in noxious emissions is attributable to useof catalytic converters through which exhaust gas streams are directed.The passage of the exhaust across a surface comprising a suitablecatalyst promotes further chemical reaction that removes a substantialfraction of the noxious CO, NO_(x), and HC substances, converting theminstead into more benign substances such as CO₂, O₂, N₂, and H₂O.Moreover, use of catalytic converters in combination withcomputer-driven, adaptive control of timing and fuel-air mixture givesan engine designer significant flexibility when optimizingengine-operating parameters to achieve reduced emissions.

[0009] Notwithstanding the market pull coming from the significantadvantages realized by interposition of catalytic converters in theexhaust stream, there remain substantial impediments to theirmanufacture. It would be desirable if converters could be manufacturedusing reliable, efficient and inexpensive construction processes; andmaintained durability and functionality over a prolonged service life.However, conventional converters fail to afford these desirablecharacteristics.

[0010] Converter constructions must produce a gas-tight enclosure sothat exhaust enters solely at an inlet port and exists exclusivelythrough an outlet port. Failure to achieve a hermetic sealingdeleteriously allows leakage of exhaust gas, circumventing thebeneficial effect of the catalyst and producing unacceptable noise. Insome cases, leakage of exhaust containing combustible gases can lead toengine backfiring and damage to other portions of the engine system.Leakage can also expose vehicle occupants to unhealthy or dangerouslevels of CO and other emissions. In addition, leaks have been known totrigger catastrophic vehicle fires.

[0011] Understandably, automobile manufacturers are impelled by severalfactors to minimize or eliminate these catalytic converter failures. Thereputation of a manufacturer as a supplier of a high-quality product isdegraded by reported failures. In addition, both market forces andcurrent U.S. environmental regulations compel an auto manufacturer towarranty the integrity and efficacy of all aspects of an auto'spollution control system. More specifically, the regulations requirethat the system function to maintain the auto's emissions withinestablished standards for an extended period of time and mileage. Anyfailures expose the manufacturer to costly warranty repairs and to theire of an inconvenienced consumer.

[0012] Heretofore, the metal housings used for catalytic converters havemostly fallen into three broad categories of construction: a “pancake”or “clamshell” form, a wrapped form, and a multipiece form, each ofwhich encloses a catalytic substrate bearing catalytically activematerial.

[0013] Typically, the “pancake” or “clamshell” form comprises stampedupper and lower shells, which are substantially identical to each other,and which have mating, peripheral, side flanges that are welded togetherto lie in a plane containing the longitudinal axis of the housing. Theyare shaped to form an internal chamber in which the catalytic substrateis mounted by “L-shaped” or other known brackets or pre-formed featuresprovided integrally in the housing component shells.

[0014] The wrapped-form housing is made with material that initially issheet-like and formed so as to generally encircle the catalyticsubstrate. This form is also known as a “tourniquet wrap,” reflectingits construction. The edges of the housing must be joined at a weldedseam that runs essentially the full axial length of the converter. Theinlet and outlet ports in this construction may either be formed as partof the wrapping operation or, more commonly, may comprise separatecomponents welded to the ends of the housing subsequent to the formationof the sheet material.

[0015] Several multipiece housing constructions are known. One formdisclosed by U.S. Pat. No. 5,118,476 comprises a tubular middle sectionin which the catalytic substrate is placed and end bushings attached toeach end of the middle section. U.S. Pat. No. 6,001,314 discloses atwo-piece housing. Each of the pieces is shaped by deep drawing toprovide an open end and a conical outer end tapered to an openingappointed for connection to associated exhaust system pipes. The twopieces are welded together with the catalytic substrate containedwithin.

[0016] Each of these multi-piece constructions must be sealed bywelding, either to close a seam in a sheet-like material or to affixappropriate end caps. The welding is needed both to provide the requiredhermetic sealing of the housing and to secure the catalytic substrate.However, each of these welds is a likely failure mode. Moreover, theOBD2 (on-board diagnostics) standard mandated by the U.S. EnvironmentalProtection Agency for passenger vehicles after 1996 imposes a furtherneed for hermetic integrity in the engine exhaust system. This standardmandates measurement of O₂ content before and after the catalyticconverter as a required input for the computerized engine controlsystem. Even a pinhole leak in the system between the sensorscompromises the accuracy of the comparison in O₂ levels which is usedfor a mass balance determination. The emissions control system fails,negating the ability of the engine control system to adaptively optimizetiming and fuel/air mixture to minimize emissions. Such failure of theemissions control system must be corrected under warrantee by thevehicle manufacturer at considerable expense and inconvenience to theconsumer.

[0017] The environment of a catalytic converter is harsh for amultiplicity of reasons, each of which can potentially cause penetratingcorrosion and ultimate failure of the converter housing. For example, aconverter used in an on-road vehicle, especially in cold climates, isexposed externally to a spray of road salt and internally to acidicexhaust gases. It is well known in the art that chemical and stresseffects combine to make weldments especially likely loci of corrosiveattack. Accordingly, a catalytic converter housing that could be formedefficiently and economically into a single piece without welding haslong been sought in the automotive art. Such a one-piece catalyticconverter housing would overcome serious shortcomings involving thereliability of extant multi-piece and welded housing forms.

[0018] The only known technique for producing single-piece housings isspinning. In this process, a catalytic element is placed within a tubeand the combined workpieces are rapidly spun about the tube'scylindrical axis while suitable tools are brought into contact with thetube at each of its ends. Sufficient deformation is thus accomplished toform tubular ports of reduced diameter at each end of the tube.Depending on the required reduction, the spinning may be carried outeither cold or hot. While the spinning approach does produce asingle-piece housing, it also carries substantial drawbacks. Theproduction forming is expensive and energy-intensive to conduct.Moreover, it results in formation of circumferential ridges on both theinside and outside surfaces of the housing in the deformed region. Theseridges are both unattractive and present significant disruption of thegas flow inside the muffler, causing turbulence and undesirable backpressure that reduce the engine power available for a given cylinderdisplacement. The magnitude of diameter reduction achievable by spinningis limited. In addition, substantial work hardening is produced in themetal in the reduced section. As a result, the ductility of the tube inthe reduced section, including the port tubulation, is too low to allowthe converter to be attached to adjoining exhaust sections by ordinaryclamps. Welded or flanged joints must be used instead. The need forwelding joints is particularly inconvenient for aftermarket and repairuse.

[0019] The spinning process is further limited by the size and shape ofproduct that it can produce. Very long shapes are unwieldy to secure andspin at the required rate in available lathes and similar machine tools.Moreover devices produced by spinning must be rotationally symmetricabout a cylindrical axis, or the resulting imbalance makes it impossibleto spin the device and accomplish the needed forming of the desiredshape. In many cases the circuitous path available for the exhaustsystem would make it highly desirable to have non-symmetric componentsavailable, such as a catalytic converter in which the inlet and outletports need not be coaxially aligned, but angulated or offset relative toone another. Such configurations cannot be formed by known spinningmethods. Furthermore, areas of the housing formed by spinning arework-hardened to an extent that renders subsequent bending and likeoperations virtually impossible.

[0020] As a result of these deficiencies, spinning is not widely used inthe manufacture of exhaust components, notwithstanding the eagerness ofthe market for a viable single-piece, seamless device which spinningmight be thought capable of producing.

SUMMARY OF THE INVENTION

[0021] In an aspect of the present invention there is provided acatalytic converter for an internal combustion engine exhaust systemhaving a single-piece, seamless metallic housing, and a method forconstructing the converter. The catalytic converter comprises: (i) atubulated gas inlet port in the housing through which exhaust gas isintroduced; (ii) a tubulated gas outlet port in the housing throughwhich the exhaust gas is discharged; (iii) a tubulated intermediatesection of the housing having an inlet end and an outlet end; (iv) aninlet transition section connecting the inlet port and the inlet end ofsaid intermediate section; (v) an outlet transition section connectingthe outlet end of the intermediate section and the outlet port; and (vi)a plurality of cascaded catalytic elements contained within theintermediate section and through which the exhaust gas passes whenflowing between the gas inlet port and the gas outlet port. The interiorsurfaces of the inlet and outlet transition sections and the gas inletand outlet ports are smooth and substantially free from ridges thereon.The inlet and outlet ports and the inlet and outlet transition sectionsare formed by swaging the ends of a seamless tube used to form thehousing. As used herein, the term “seamless metallic tube” is understoodto mean a generally cylindrical metallic tube produced, e.g. byextrusion, or a tube formed by shaping a long, relatively narrow sheetinto a tube like structure by bringing opposite, generally paralleledges of the sheet into abutment and joining the edges by welding.

[0022] Exhaust gas produced by operation of the engine passes into theconverter and through the catalytic element. Noxious substances in theexhaust, including CO, NO_(x), and incompletely combusted hydrocarbonsare converted to more benign substances through the action of thecatalytic element, which preferably comprises a frangible ceramichoneycomb structure having a plurality of internal passages coated withone or more catalytically active substances.

[0023] Advantageously, the one-piece, seamless converter construction ofthe invention is economical to produce and eliminates welding of housingcomponents that are highly prone to failure. More specifically, theone-piece, seamless construction of the converter housing reduces thesize and number converter parts (as compared with known practicalconstructions) while, at the same time, increasing the converter'seffectiveness and improving its construction and manufacture. Theconverter is inherently economical to produce and can bemass-manufactured in the large volumes required to supply originalequipment converters directly to manufacturers of automobiles and trucksfor factory installation in exhaust systems. Furthermore, theimprovement of the housing affords additional advantages includingbetter vehicle fuel efficiency and better manufacturing economics.

[0024] The elimination of any welding of seams or end caps afforded bythe one-piece construction of the present catalytic converter greatlyenhances its reliability during service. Welds are notoriouslyvulnerable as loci of corrosive attack due both to chemical and stresseffects. In operation, catalytic converters in typical on-road vehicleapplications are exposed externally to road salt and other corrosivesubstances and internally to acidic exhaust gases. The absence ofweldments in the present catalytic converter removes a prime source offailures during the service life of the device.

[0025] The manufacture of the catalytic converter of the inventioncomprises use of swaging processes to form the tubulated ends of thecatalytic converter. The central section of the housing, being generallylarger in inner dimension than the ends, envelops and holds thecatalytic element in position. Swaging is used in the practice of thisinvention generally to reduce the ends of the housing to a tube diameterappointed for connection of the converter to the adjoining components ofthe engine exhaust system.

[0026] Compared to other known methods used to form ends, swaging oftenaffords a number of advantages. The reduction in diameter can be carriedout in a sequence of steps using a plurality of dies. As a result, theswaging process is highly adaptable, providing a designer with greatflexibility in tailoring a housing to fit into an available space and inoptimizing the dynamics of gas flow through the device. The tubulatedends and transition sections are smooth and substantially free ofcircumferential ridges on either the inside or outside surfaces. Theexternal appearance of the device is enhanced. The smoothness of theinside surface and the absence of ridges thereon minimizes generation ofturbulence that impedes the flow of exhaust gas through the converterand causes excessive backpressure. Swaging ordinarily maintains a levelof ductility sufficient to allow the converter to be connected to therest of the exhaust system by clamped, welded, or flanged joints.

[0027] A further benefit offered by some embodiments of the presentmethod over spinning is the ability to form more intricate structures,including those having features such as extended length and bends orother non-symmetrical configuration. Since the present end-formingprocess does not entail rapid rotation of the workpiece, neitherrotational balance nor the size of available lathes or similar machinetools is a consideration. End forming can thus produce converters havinginlet and outlet ports and transitional sections which can be round orhave a variety of other shapes, such as that of ovals, and especially“flat ovals”, needed to accommodate vehicle clearance or other size andspace requirements. A converter could also be formed with bends in anyof its sections. Designs can also be formed wherein the catalyticelement is an oval cylinder instead of a right circular cylinder.

[0028] In one aspect of the invention the catalytic element comprises afrangible ceramic honeycomb structure having a plurality of passagesextending therethrough. The surfaces of the passages are substantiallycovered with a finely dispersed, catalytically active substance. Thisconstruction makes effective use of the catalytic substance, whichgenerally comprises one or more of the expensive platinum-group noblemetals including Pd, Pt, and Rh. The ceramic honeycomb is preferablyencircled with a mat of an intumescent material which acts toresiliently and insulatively secure it within the inside diameter of theintermediate section. The intumescence of the mat material creates areliable and gas-tight seal that protects the ceramic during itsconstruction and in-service life. In addition, the intumescent materialforces the exhaust gas stream to pass through the passages of thehoneycomb, maximizing catalytic efficacy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The invention will be more fully understood and furtheradvantages will become apparent when reference is had to the followingdetailed description of the preferred embodiments of the invention andthe accompanying drawings, wherein like reference numerals denotesimilar elements throughout the several views and in which:

[0030]FIG. 1 is a perspective view of a catalytic converter inaccordance with the invention;

[0031]FIG. 2 is a longitudinal, cross-sectional view along the axialmid-plane of the catalytic converter depicted by FIG. 1;

[0032]FIG. 3 is a longitudinal, cross-sectional view along the axialmid-plane of a catalytic converter in accordance with another aspect ofthe invention;

[0033]FIG. 4 is an axial cross-sectional view taken in the intermediatesection of the converter depicted by FIG. 3;

[0034]FIG. 5 is a fragmentary longitudinal cross-sectional view takenalong the axial mid-plane of a catalytic converter having pluralcatalyst elements in accordance with another aspect of the invention;

[0035]FIG. 6 is a fragmentary longitudinal cross-sectional view takenalong the axial mid-plane of a catalytic converter having pluralcatalyst elements of different diameters and disposed in differentintermediate subsections in accordance with another aspect of theinvention;

[0036]FIG. 7A is a fragmentary longitudinal cross-sectional view takenalong the axial mid-plane showing an intermediate stage in one processfor fabricating the housing of the converter depicted by FIG. 6;

[0037]FIG. 7B is a fragmentary longitudinal cross-sectional view takenalong the axial mid-plane showing an intermediate stage in an alternateprocess for fabricating the housing of the converter depicted by FIG. 6;

[0038]FIG. 8A is a fragmentary longitudinal cross-sectional view takenalong the axial mid-plane showing an intermediate stage in one processfor fabricating the housing of the converter depicted by FIG. 6;

[0039]FIG. 8B is a fragmentary longitudinal cross-sectional view takenalong the axial mid-plane showing an intermediate stage in an alternateprocess for fabricating the housing of the converter depicted by FIG. 6,the view of FIG. 8B taken subsequent to the stage seen in FIG. 8A.

[0040]FIG. 9A is a perspective view, partially cut away, of a tubepreform engaged by gripping dies during production of one implementationof the present method for producing a catalytic converter; and

[0041]FIG. 9B is a horizontal longitudinal cross-sectional view of thetube preform engaged by gripping dies also shown in FIG. 9A, thecross-section taken at level IX-IX of FIG. 9A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] The present invention is directed to a catalytic converter, whichterm is herein used to mean a catalytic converter, muffler,pre-catalytic converter, or the like, incorporated in the exhaust systemof an internal combustion engine. Generally stated, the catalyticconverter is housed in a single-piece, seamless metallic housingpreferably made of a corrosion-resistant metal alloy such as a stainlesssteel alloy. The housing has tubulated gas inlet and outlet portsthrough which exhaust gas is introduced and discharged, respectively.The converter also comprises a plurality of cascaded catalyst elementscontained within a tubulated intermediate section of the housing. Duringits passage through the converter, exhaust gas is caused to flow overthe surface of the catalyst element. A catalytically active materialpresent on the catalyst element promotes chemical reactions that purifythe gas stream. This is accomplished by converting certain chemicalspecies therein to other species which are generally considered notharmful to life or the environment, or which present a significantlyreduced danger.

[0043] Referring now to FIGS. 1 and 2 there is shown generally at 10 acatalytic converter comprising one aspect of the invention. Thecatalytic converter 10 has a one-piece, seamless metal housing composedof a corrosion-resistant metal, preferably stainless steel. Theembodiment depicted by FIG. 1 is generally cylindrical, having a roundcross-section at each point along its axial length. However, in otherembodiments the housing may have a non-circular cross-section, such as aflat oval. Housing 12 is generally tubular in shape, and is providedwith tubulated gas inlet port 14 having inlet orifice 16 and tubulatedgas outlet port 18 having outlet orifice 20. Converter 10 is appointedfor use in the exhaust system of an internal combustion engine (notshown). Inlet port 14 is connected to tubulated intermediate section 22of housing 12 by inlet transition section 24. Intermediate section 22 isconnected to outlet port 18 through outlet transition section 26.Exhaust gas produced during operation of the internal combustion engineenters the converter through inlet orifice 16 in inlet port 14. Theexhaust gas then passes successively through inlet transition section24, intermediate section 22, and outlet transition section 26, beforebeing discharged through outlet orifice 20 of outlet port 18. Inlet andoutlet ports 14, 18 of converter 10, though of reduced diameter owing tothe manufacturing process described hereinafter, are not work hardened.As a result, the ports, 14, 18 of converter 10 can be readily connectedto other components of the exhaust system by welded, clamped, andflanged joints produced by suitable means known in the engine art.

[0044] The arrangement of the components inside the catalytic converter10 of FIG. 1 is best seen in the cross-sectional view of FIG. 2.Converter 10 further comprises catalytic element 28. In an aspect of theinvention catalytic element 28 comprises a generally cylindrical,frangible, heat-resistant ceramic honeycomb substrate 30 having a largenumber of passages 32 therethrough, each of the passages 32 runninggenerally parallel to the cylindrical axes of both substrate 30 andintermediate section 22. Passages 32 are generally arranged in ahoneycomb pattern or a similar array. Passages 32 extend completelythrough substrate 30, thereby allowing the passage of gas from one end34 of element 28 to the other end 36 thereof. In an aspect of theinvention substrate 30 is composed of cordierite. Other ceramicmaterials having adequate strength, thermal shock resistance, heatresistance, and chemical compatibility both with the exhaust gas streamand catalyst material applied thereto may also be used. The interiorsurfaces of passages 32 are substantially covered with a substancecatalytically active to induce the chemical reaction of substancespresent in engine exhaust for the purification thereof. In some aspectsof the invention, catalytic element 28 may comprise a plurality ofceramic or other substrates arranged sequentially or in parallel, e.g.to achieve better gas flow, to provide more than one type of catalyst,or to facilitate manufacture of a longer converter. Sequentiallydisposed substrates may be in substantially abutting relationship or maybe separated by a free space which may be desired for optimizing gasflow.

[0045] One skilled in the art will also appreciate that while theembodiment shown in FIGS. 1 and 2 is cylindrically symmetric, theinvention also contemplates aspects in which bends, angulations, andother non-symmetric features may be present. The intermediate sectionmay be oval or “pancake” shaped and have a catalyst element of matingshape therein. The inlet and outlet ends, as well as the intermediatesection, may be coaxially directed, as shown in FIGS. 1-2.Alternatively, the housing may be structured in such a way that the endsmay be directed along parallel but offset directions, or at a relativeangle.

[0046] In addition, the inlet and outlet ends may be extended to serveas portions of the engine exhaust system. While this extension entailsadditional forming steps, the corresponding reduction in the number ofparts in the total engine exhaust system is advantageous both forsimplifying assembly and for eliminating otherwise required joints thatare also prone to corrosion failure.

[0047] The present catalytic converter employs any of a variety of knowncatalytically active substances. Many of these substances contain noblemetals such as Pt, Pd, and Rh. As a result of the needs to (i) use thesehigh cost materials as efficiently as possible and (ii) maximize thecatalytic surface's effective area, the catalyst material is generallyprovided in a finely divided form coated on, and adhered to, a catalyticsubstrate. Suitable catalyst materials include three-way catalystsappointed to convert nitrous oxides, carbon monoxide, and hydrocarbonsto nitrogen, water, and carbon dioxide. Means for selecting a suitablecatalyst material and for ascertaining the specific activity and theeffective surface area of a catalytic material are well known in thecatalyst art. The converter may also comprise an oxidation catalyst andmeans (not shown) for supplying secondary air to intermediate section 22so as to promote conversion of carbon monoxide and hydrocarbons to waterand carbon dioxide. Other forms of catalyst substrate, includingmetallic catalyst support systems, may also be used in the practice ofthe present invention.

[0048] Ceramic substrate 30 is preferably supported and held securely insealing contact within intermediate section 22 by a generally encirclingmat 38. The mat is preferably resilient, insulative, and shock absorbentand may be composed of a gas impervious, vermiculite based material,available in the open market. Such a material is intumescent, that is,it expands substantially upon heating. Typically a mat having an initialthickness of approximately ¼″ is used. During assembly of converter 10,mat 38 is wrapped to substantially encircle the ceramic substrate 30 andcover at least a portion of its longitudinal or axial length.Preferably, a short portion of the cylindrical periphery of the ceramicsubstrate 30 at each of its ends 34, 36 is left uncovered by mat, asdepicted by FIG. 2. Preferably, the axial length uncovered at each endranges from about half to three times the thickness of the mat. Thisholdback ensures that portions of the mat do not break off and block anyof the passages through honeycomb structure 30. The wrapped mat 38 iscompressed radially to a thickness approximately half of its initialthickness prior to insertion of the combined substrate 30 and mat 38into the inner diameter of intermediate section 22 to secure thepositioning of the combined assembly within the converter housing 12.

[0049] One form of mat suitable for the practice of the inventioncomprises a flexible intumescent sheet, which may be used for mountingautomotive catalytic converter monoliths. The sheet comprises anunexpanded vermiculite produced by subjecting vermiculite ore containinginterlamellar cations to a potassium nitrate solution for a timeinterval sufficient to ion exchange interlamellar cations within the orewith potassium ions; an inorganic fibrous material; and a binder. Thesheet material may be provided with, or temporarily laminated to, abacking sheet of kraft paper, plastic film, non-woven synthetic fiberweb, or the like. Another suitable form of intumescent sealing mat 38 isavailable on the market under the trade name of “3M INTERAM MAT.”Suitable intumescent sealing mat is also sold commercially by Unifrax astype “XPE.”

[0050] Advantageously the intumescence of mat 38 ensures the securemounting of catalytic substrate 30 within the converter housing. Inaddition, the intumescent mat 38 seals the gap between the outsidecylindrical surface of catalytic substrate 30 and the inside cylindricalsurface of intermediate section 22. This “sealing action” forcessubstantially all the gas flowing into the converter 10 through inletport 16 to pass through internal passages 32 in substrate 30 beforeexiting through outlet port 18. Efficacy of the catalytic materialcoating the surface of internal passages 32 is thereby enhanced, sinceexhaust gas is maximally exposed to the catalytically active material.The resiliency of mat 38 serves to cushion and protect frangiblesubstrate 30 during the initial fabrication of converter 10. Moreover,mat 38 retains this resiliency even after repeated thermal cycling. Theconstituents of converter 10 experience differential thermal expansionand contraction during each cycle of heatup, operation, and cool-down ofthe engine wherein exhaust passes through converter 10. By virtue ofmaintaining its resiliency, mat 38 is able to protect substrate 30 fromdamage due to this pattern of differential expansion, while stillmaintaining an adequate degree of sealing during the entirety of eachoperating cycle.

[0051] While the aspect of the invention depicted by FIG. 1 comprises acatalytic converter employing a catalyst dispersed and supported on aceramic substrate, other forms of catalyst and substrate may also beused in the practice of the invention. For example, a catalyst may bedispersed on a metallic substrate, which may take the form of corrugatedmetal foil that is coiled upon itself to form a generally cylindricalstructure having a plurality of passages longitudinally extendingtherethrough. Since metals are substantially less frangible and tougherthan known ceramic catalyst substrates, a metal catalytic support ofthis form may allow elimination of intumescent material 38 in theconstruction of converter 10.

[0052] A variety of metallic alloys are suitable for the housing of theinvention. Alloy selection is made on the basis of cost and the level ofperformance required with respect to temperature capability, durability,and corrosion resistance needed. For automotive use, ferritic stainlesssteels are commonly used, including various 400-series alloys. Mostcommonly 409SS and SAE51409 alloys are employed. For small engines,including non-propulsion applications, and other instances where costconsiderations are dominant, carbon steels are frequently used. Fordemanding applications wherein especially long life and high corrosionresistance are desired, such as marine engines, austenitic and300-series stainless steel alloys such as 304SS are generally employed.

[0053] The catalytic converter of the invention may be manufactured in awide range of sizes to accommodate the requirements of differentengines, which may range from small engines of a few horsepower or less,such as might be used in lawn mowers, snow blowers, weed trimmers, andsimilar off-road, non-propulsion applications, to large diesel enginesused in trucks. Many present automotive applications employ ceramicsubstrates commercially available in standard diameters of 25 and 4inches. Automotive converters are typically about 9 to 20 inches intotal length. Small, non-propulsion engines may employ substrates assmall as 1-inch diameter or even less and may be only a few inches long.The minimum length of the substrate is generally determined by the flowvelocity and the amount of time the exhaust gas must be at the catalyticsurface to achieve a chemical reaction that sufficiently reduces theconcentration of pollutants. The total length of the converter is thendetermined both by the length of the active catalyst and the requiredshaping of the transition sections and the inlet and outlet ports. Oftenthe inlet and outlet ports of a converter are approximately half thediameter of the substrate to reduce exhaust flow impedance. The taperingof the inlet transition and outlet transition sections may range fromvery gradual to very abrupt. For automotive applications the transitionsections are often chosen with a straight taper of 10-15° forsimplicity. However, curved transitions provided in some implementationsof the present converter are generally advantageous for betteraerodynamics. In one aspect of the invention, the intermediate sectionis cylindrical and the length of the inlet and outlet transitionsections is chosen so that each length ranges from about 30% to 100% ofthe diameter of the intermediate section.

[0054] The catalytic converter shown in FIGS. 1-2 and describedhereinabove is of the so-called under-floor type normally disposed underthe floor of an automotive vehicle for reasons of available space.However, it will be understood by one skilled in the art that theprinciple of the present invention may be applied to catalyticconverters of many different types, including those types appointed tobe mounted proximate the exhaust manifold of an internal combustionengine. These types are sometimes denoted as pre-cat catalyticconverters if mounted within about thirty-six inches of the manifold oras pre-light catalytic converters if mounted within about eight inchesof the manifold. Additionally, it will be appreciated that the principleof the present invention may be applied to the manufacture of catalyticconverters for a variety of internal combustion engines other than thoseof an automotive vehicle, and converters that are integrated withmufflers, resonators, or other similar components in the exhaust system.

[0055] In accordance with an aspect of the invention, the inlet andoutlet transition sections and the inlet and outlet ports of thecatalytic converter are formed by swaging. In the aspect of theinvention depicted in FIGS. 1-2, the converter housing 12 is formed froma cylindrical metallic tube, the diameter of which is uniform throughits length. The ends of the tube are swaged to form the transitionsections 24, 26 and the ports 14, 18 with mat 38 and substrate 30 beinglocated in intermediate section 22. As depicted by FIG. 2, the insidediameter of intermediate section 22 through substantially its entirelength is d_(t). A swaging operation forms transition section 24 andinlet port 14, tapering the tube smoothly to a diameter of d_(i) in theinlet port region, so that inlet port 14 may be attached to thepreceding component of the exhaust system. A swaging operation is alsoused to form outlet transition section 26 and outlet port 18. Thediameter d_(o) of outlet port 18 may be the same as diameter d_(i) ofinlet port 14. Alternatively, different diameters may be selected toaccommodate the requirements of the exhaust system.

[0056] In FIGS. 3 and 4 there is shown an aspect of the inventioncomprising a variation of the form of the housing 12 depicted by FIGS. 1and 2. In this aspect, a further forming operation, which may includeswaging, is optionally applied to intermediate section 22 of housing 12to form therein a plurality of rib-like indentations 50, which areaxially elongated along intermediate section 22. Preferably indentations50 extend along a substantial portion of intermediate section but do notextend to inlet and outlet transition sections 24, 26. Thecross-sectional view of FIG. 4, taken at lines A-A shown by FIG. 3,depicts an aspect in which three such indentations are present.Preferably indentations 50 extend radially inward to a depth of at mosthalf the thickness of intumescent mat 38. They act to further supportand constrain mat 38 and substrate 30 from axial movement and to assuresealing of the mat/substrate assembly within the walls of housing 12.Indentations 50 should not be so deep as to cause fracture of substrate30.

[0057]FIG. 3 also depicts optional circumferentially extendingindentations 52 at the junction between inlet transition section 24 andintermediate section 22 and at the junction between intermediate section22 and outlet transition section 26. These indentations 52 serve tosupport and constrain substrate 30 from moving axially. If present,these indentations are preferably of a depth less than or about equal tothe thickness of the mat. This indentation in some cases alsoadvantageously improves the uniformity of gas flow across presented atthe face area of substrate 30. In other embodiments a screen, baffle, orsimilar structure may also be used to protect the inlet and outlet ofthe ceramic substrate from the intrusion of foreign matter and toimprove gas flow.

[0058] The swaging used to form each of the ends of converter housing 12is carried out by mechanically forcing a suitably configured die overeach end of the tube, the force being applied in a generally axialdirection and the die being designed to cause flow of the metal andthereby reduce the tube's diameter while maintaining its circularity.Preferably, the swaging is carried out in a plurality of swaging stepsusing a plurality of dies to form each end of the converter housing 12.A suitable sequence of dies may further be used to achieve a desiredprofile in the transition section. The profile may be as simple as acone, but alternatively comprises a more complicated pattern having acombination of curvatures in which there is no discontinuity in theslope of the inside surface of the converter in its axial direction. Insome embodiments of the invention both ends of the housing are swagedsimultaneously by applying oppositely directed compressive forces todies on each end using hydraulically driven rams.

[0059] The flexibility in choosing the profile of the transitionsections of the present converter is highly advantageous. The profile ischaracterized, in part, by the curvature at each point along the lengthof the transition sections and the overall rapidity with which thesections taper. A converter designer must satisfy several constraints.The overall length of the converter may be limited to a maximum lengthby available space. The required gas flow rate, allowable impedance togas flow, and required extent of exposure of the exhaust to activecatalyst, along with available forms of substrate and the geometry ofthe passages therethrough, generally constrain the area and lengthrequired for the catalytic element. In addition, properly designedtransition sections advantageously maintain a generally laminar flow ofexhaust gas and minimize the generation of unwanted turbulence therein.This turbulence undesirably increases the impedance of the catalyticconverter and results in excessive backpressure. The absence of theabove-mentioned discontinuity in slope in the transition sections,together with suitable transition profiles with gradually tapereddiameters, minimizes this turbulence.

[0060] A further benefit of swaging over other forming techniques, suchas spinning, is its ability in most cases to produce reduced sectionswhile maintaining smoothness of both the interior and exterior surfacesof the converter. When properly carried out, significant reductions indiameter can be made without producing internal and externalcircumferential ridges that are typically produced by spinning. Ifpresent, such ridges on the interior surface of an exhaust component area further source of undesirable turbulence, as previously discussed. Inaddition, the swaging technique ordinarily results in an exteriorsurface that is smooth and aesthetically appealing, minimizing itsvulnerability to pitting, corrosion, or like attack, and rendering iteasily finishable by plating or other coating operations. The swagingtechnique also produces a smooth inner surface without significantridges.

[0061] A further disadvantage of techniques such as spinning is theamount of work hardening that results in the reduced section. This workhardening is highly detrimental in that it leaves the inlet and outletports with a ductility level that is insufficient to allow the ports tobe reliably and hermetically attached to other exhaust components byclamping. Instead, joints have to be made using welds or flanges, whichare more expensive and difficult to implement, especially withafter-market installations.

[0062] The present catalytic converter may be manufactured in otherconfigurations. For example, the converter may incorporate a pluralityof cascaded catalytic elements. FIG. 5 depicts such an embodiment 11including two catalyst elements 28, 28′ in a generally cylindricalconverter. Each of the elements is of substantially similar size and isencircled by an intumescent mat 38. The elements are laterally spaced bya distance s₁. Preferably, s₁ is at least 10% of diameter d_(t). Exhaustgas enters through inlet port 14 and transits elements 28 and 28′,sequentially entering end 34 of element 28 and exiting at end 36,transiting the space between the elements, and then entering end 34′ ofelement 28′ and exiting at end 36′, and finally exiting the converterthrough outlet port 18. In some embodiments, the elements 28, 28′include substantially the same catalytically active material, which maybe a coating applied to a ceramic honeycomb substrate such ascordierite. However, the elements may also incorporate differentcatalytic substances, which may be optimized to catalytically promotedifferent reactions. For example, one catalyst may be of a type used toconvert NO_(x) to N₂+O₂, while another catalyst may be of a type used tocompletely combust unburned HC. Other combinations, and configurationsemploying more than two elements, are also understood to be within thescope of the invention.

[0063] In addition, the various catalytic elements may be of differentdiameters. One such configuration 70 employing three catalytic elementsis seen in FIG. 6. In this embodiment, the intermediate section of thecatalytic converter includes subsections 72, 74, and 76, in whichcatalytic elements 78, 82, and 84, respectively, are disposed. Thesubsections are joined by transitions 73, 75. The embodiment shown alsoincludes intumescent mats 80 encircling and securing each of thecatalytic elements in their respective subsections. The subsectionssequentially decrease in diameter from transition inlet end 90 to outletend 91. In other embodiments (not shown), one or more of the transitionsections 73, 75 may include a bend, so that ends 90 and 91 may benon-coaxial or offset from one another. Such shapes are advantageouslyemployed in some vehicle applications, wherein the exhaust system mustfollow a circuitous route due to the lack of any available straight pathfrom the engine to the desired terminus of the exhaust system.

[0064] The ability to employ plural catalysts and to configure them onsubstrates of different diameters provides significant designflexibility in providing maximum catalytic efficacy. The differentmaterials can be selected to promote different reactions that addressthe multiple undesirable constituents in a typical exhaust stream. Bychanging the geometrical configuration of the overall converter and theindividual catalytically active elements, the flow pattern and resultingexhaust gas residence time can also be beneficially optimized. Thegeometric flexibility is also beneficial in vehicle designs in which theavailable space for locating the converter and other exhaust systemcomponents is often limited and circuitously disposed in the vehicleundercarriage.

[0065] Multi-stage converter configurations, such as that depicted byFIG. 6, may be manufactured using a number of manufacturing techniques,which may be carried out in a variety of orders. The housing with itsintermediate subsections of the requisite diameters can be formed beforeor after the different catalyst elements are inserted.

[0066] In one implementation usable to make the housing depicted by FIG.6, there is provided a preform having the approximate diameter of thelargest subsection 72. As shown generally at 70′ in FIG. 7A, swaging isthen used to reduce the diameter of a portion of the preform long enoughto accommodate sections 74 and 76, thereby creating temporary subsection74′, with the concomitant formation of transition 73 of the ultimatelydesired shape. A subsequent swaging operation (not shown) is applied toa portion of temporary subsection 74′ to form the ultimately desiredsubsections 74 and 76, with transition 75.

[0067] Alternatively, as seen generally at 70″ in FIG. 7B, a preformwith the approximate diameter of the largest subsection 72 is againprovided. Subsection 76 is first formed, creating temporary subsection72″ and transition 75″. A subsequent swaging is then carried out to formsubsection 74 and reshape temporary transition 75″ into transition 75having the desired shape, as shown in FIG. 6.

[0068] Either of the formations shown in FIGS. 7A-7B may be carried outbefore or after insertion of the catalytic elements. Other sequences mayalso be employed. For example, the formation of subsections and theinsertion of the associated catalytic elements can be carried out inalternation. Such a sequence is also preferably employed if thetransitions include bends. It is also possible to create the transitionsbetween subsections by other forming methods such as spinning. However,the aforementioned swaging is preferred, since it ordinarily produces aninner surface that is smooth and substantially ridge-free, whereby flowdisruption is minimized. Also, a housing that is not cylindricallysymmetric is far easier and more efficient to form using swaging thanspin forming or other similar processes.

[0069] In an aspect of the method of the invention the forming processis carried out semicontinuously. Seamless tubes having substantially thediameters desired for the intermediate section (or the largest of itssubsections) are supplied in long lengths. From these tubes are cutpreforms adapted to be formed into the catalytic converter housing.Ceramic substrates are provided having the desired diameter and length.Each substrate is wrapped with an intumescent mat to form the catalyticelement. The mat is compressed and the combined assembly inserted intothe free end of the supply tube. The supply tube is then grippedcircumferentially and its free end is swaged to form the inlet port andinlet transition section. Subsequently, the supply tube is cut to removea portion thereof having a requisite preselected length, therebydefining the preform. The preform is then re-gripped and the other endswaged to form the outlet port and outlet transition section. Most ofthe steps of this process are easily automated to provide a high levelof manufacturing efficiency and process control, thereby minimizingproduction costs.

[0070] In another aspect of the method of the invention, preforms arecut to length from a seamless tube having substantially the diameterdesired for the intermediate section of the converter. A ceramicsubstrates having the requisite length is wrapped with an intumescentmat to form the catalytic element and inserted approximately in themiddle of each preform. The preform, with its substrate, is then placedin a hydraulic ram assembly having swaging dies coaxially aligned andadapted to be forced compressively over each of the preform's ends,thereby simultaneously swaging each end. A plurality of dies aresequenced into a hydraulic ram and used stepwise to accomplish thedesired reduction to the final inlet and outlet port diameters and toform the inlet and outlet transition sections. Alternatively, aplurality of rams are used, with the preform being indexed between rams,which are appointed with a desired sequence of dies to progressivelyswage the ends in a plurality of graduated swaging stages.

[0071] One method by which the catalytic converter partially depicted byFIG. 6 may be produced is further elucidated by reference to FIGS. 8A-B.There is provided a tube preform having approximately the diameter ofthe largest subsection 72 of the cascaded arrangement of intermediatesection 70. Catalytic substrate 78 encircled by intumescent mat 80 isfirst inserted into section 72. Thereafter, an end of the preform isend-formed by forcibly inserting the preform into swaging die 92 adaptedto reduce the diameter of the preform to create temporary subsection 74′and transition 73. Then the workpiece is removed from die 92 andcatalytic substrate 82 encircled by intumescent mat 80 is disposed intemporary subsection 74′ at a preselected location. The preform, nowbearing substrates 78 and 82, is again end-formed using swaging die 94,which reduces the diameter of part of the preform to the sizepreselected for section 76 and creates transition section 75. In stillanother step (not shown), substrate 84 and encircling intumescent mat 80are inserted in the preform. Both ends of the preform are swaged toprovide inlet and outlet ports. It will be recognized that while theembodiment of the invention depicted by FIGS. 6-8 employs three cascadedcatalytic elements, embodiments with other numbers of catalyticelements, ranging from two to four or more elements are also possibleand are to be understood as being within the scope of the presentinvention. As noted above, the housing, including the intermediatesection and subsections graduated in diameter may be formed prior toinsertion of the catalytic elements into the respective subsections, orthe forming and insertion operations for each subsection may beaccomplished in alternation, as set forth above. Implementations inwhich the individual forming and insertion operations are alternatelyaccomplished also permit bends or offsets to be formed between thesubsections. They also permit formation in which the sizes of thevarious subsections do not increase or decrease in strict sequence.However, configurations having sequentially decreasing diameters of thesubsections, such as that depicted in FIG. 6, are especially preferredand simpler to form.

[0072] It will be understood that swaging operations are ordinarilycarried out with part of a tube preform secured by grips so thatsufficient axially-directed force may be applied to form at least one ofthe tube ends into the desired configuration, although methods in whichthe inlet and outlet ends are formed simultaneously may rely on theoppositely directed axial swaging forces to eliminate the gripping orreduce the amount of gripping force needed.

[0073] In some implementations of the present method, the grippingoperation is accomplished by gripping dies that substantially encirclethe tube and provide radially directed force to reduce the diameter ofan inner section of the preform. One such operation is shown generallyat 94 by FIGS. 9A-B. Two gripping dies 96 are disposed to surroundpreform 98. The dies 96 are urged together by force radially directed atF, as seen in FIG. 9B. Dies 96 are adapted to create reduced diametersection 100 joined to the remainder of tube 98 by transition sections102. In the embodiment shown, the reduction is carried out withcatalytic element inserted in the section to be reduced. The element maycomprise ceramic substrate 104 with encircling intumescent mat 105, asdepicted, or any other form of catalytic element. Although twocomplementary dies are shown, other embodiments may employ three or morecomplementary dies which collectively surround tube 98 and, when forcedradially, cause a substantially cylindrical reduction of the tubediameter. The force used to compress dies 96 is ordinarily providedhydraulically, but other known sources of mechanical force may also beused.

[0074] The use of gripping dies that simultaneously reduce a tubediameter beneficially improves the efficiency of a tube fabricationprocess by eliminating a step in many known forming processes. Inparticular, gripping operations capable of opposing the substantialaxial forces required are frequently required for successful endforming. Heretofore, a separate step for reducing the diameter of anintermediate portion of the tube has been required. However, the use ofdies that accomplish both the required gripping and reduction eliminatesa process step.

[0075] The following example is presented to provide a more completeunderstanding of the invention. The specific techniques, conditions,materials, proportions and reported data set forth to illustrate theprinciples and practice of the invention are exemplary and should not beconstrued as limiting the scope of the invention.

EXAMPLE 1 Preparation and Testing of a Catalytic Converter

[0076] A catalytic converter of a type commonly used for emissionsreduction from an automobile engine was formed using a catalytic elementcomprising a conventional platinum-group noble metal type of catalystsupported on the walls of multiple passages extending through acordierite ceramic honeycomb structure. The honeycomb structure wasabout 3.6 inches in diameter and 3 inches long. The housing for theconverter was formed from a seamless tube of 409 stainless steel alloyhaving an outside diameter of about four inches and 0.049-inch walls.The honeycomb structure was wrapped with an intumescent mat compressedto about 0.12 inches thickness, and the combined mat and ceramic werethen inserted into the center part of the generally tubular converterhousing. The ends of the housing were then end-formed to provide aninlet port section and an outlet port section, each of which having adiameter of two inches and a length of two inches. Tapered transitionsections about two inches long joined the inlet and outlet sections tothe central section.

[0077] The converter was subjected to standard tests known in theautomotive industry and conformed to procedures set forth in 40 CFR85.2116. The following tests were conducted.

[0078] The expansion of the intumescent mat material was tested by adial-gage technique. The mat was heated to a maximum temperature of 825°C. and its expansion was measured and found to exhibit a maximum ofabout 120-130% relative to its initial thickness. This value is wellwithin the usual range known in the art to result in satisfactorysealing and positioning of a ceramic substrate in a catalytic converterhousing.

[0079] The cold push-out resistance of the ceramic substrate wasmeasured by a standard mechanical pressing technique. The experiment wascarried out by gripping the converter housing and exerting mechanicalforce through an arbor pressing axially on the substrate. It was foundthat a force of about 60-80 pounds was required to initiate shear of theintumescent mat and cause displacement of substrate. The observed forcewas well within standards recognized in the art, in which movement atless than about 30 pounds indicates an inadequately supported and sealedsubstrate, while movement at over about 200 pounds is indicative ofexcessive engagement of the substrate by the intumescent mat that islikely to result in chipping or brittle failure of the substrate duringconverter use.

[0080] The catalytic efficiency of the converter was measured by a sweeptest using gas chromatographic analysis of the conversion efficiency forknown CO, HC, and NO_(x) pollutants to benign substances. Minimalbackpressure was observed, confirming the adequacy of flow through theconverter, including the catalytic substrate. Satisfactory catalyticefficiency performance was observed, indicating the suitability of theconverter for use in an automotive application that achieves compliancewith applicable emissions standards.

[0081] The results of the foregoing tests establish that a one-piece,seamless converter satisfactorily demonstrates performance sufficient tomeet requirements for emissions mitigation in an automotive application.

[0082] Having thus described the invention in rather full detail, itwill be understood that such detail need not be strictly adhered to butthat various changes and modifications may suggest themselves to oneskilled in the art, all falling within the scope of the presentinvention as defined by the subjoined claims.

What is claimed is:
 1. A catalytic converter for an internal combustionengine exhaust system, comprising: a. a single-piece, seamless metallichousing; b. a tubulated gas inlet port in said housing through whichexhaust gas is introduced; c. a tubulated gas outlet port in saidhousing through which the exhaust gas is discharged; d. a tubulatedintermediate section of said housing having an inlet end and an outletend; e. an inlet transition section connecting said inlet port and saidinlet end of said intermediate section; f. an outlet transition sectionconnecting said outlet end of said intermediate section and said outletport; and g. a plurality of cascaded catalytic elements contained withinsaid intermediate section and through which said exhaust gas passessequentially when flowing between said gas inlet port and said gasoutlet port.
 2. A catalytic converter as recited by claim 1, whereinsaid interior surfaces of said inlet and outlet transition sections andsaid gas inlet and outlet ports are smooth and substantially ridge-free.3. A catalytic converter as recited by claim 1, wherein each of saidcatalytic elements comprises: a. a ceramic substrate having a pluralityof passages extending therethrough; and b. a catalytically activematerial present on a substantial portion of the surface of each of saidpassages.
 4. A catalytic converter as recited by claim 3, furthercomprising an intumescent mat encircling each of said ceramic substratesand sealing the external surface of said ceramic substrates to the innersurface of said intermediate section.
 5. A catalytic converter asrecited by claim 1, wherein said metallic housing is composed of astainless steel alloy.
 6. A catalytic converter as recited by claim 1,wherein said intermediate section is substantially cylindrical and hasdiameter, each of said inlet and outlet transition sections has alength, and said length of each of said inlet and outlet transitionsections ranges from about 30% to 100% of said diameter of saidintermediate section.
 7. A catalytic converter as recited by claim 1,wherein said inlet and outlet transition sections and said gas inlet andoutlet ports have been formed by swaging.
 8. A catalytic converter asrecited by claim 7, wherein said swaging is carried out using aplurality of swaging steps.
 9. A catalytic converter as recited by claim7, wherein said intermediate section has a plurality of rib-likeindentations axially elongated along said intermediate section.
 10. Acatalytic converter as recited by claim 1, wherein said plural catalyticelements have substantially the same diameter.
 11. A catalytic converteras recited by claim 1, wherein said plural catalytic elements havedifferent diameters.
 12. A catalytic converter as recited by claim 1,wherein said plural catalytic elements contain different catalyticallyactive materials.
 13. A catalytic converter as recited by claim 1,wherein said intermediate section has a plurality of subsections, eachsubsection containing at least one of said catalytic elements.
 14. Acatalytic converter as recited by claim 1, said elements being spaced bya distance of at least about 10% of the diameter of said elements.
 15. Acatalytic converter as recited by claim 13, wherein said subsections andsaid plural catalytic elements have different diameters.
 16. A catalyticconverter as recited by claim 1, wherein said inlet and outlet ports andsaid subsections of said intermediate section are not coaxial.
 17. Aninternal combustion engine system having an exhaust system comprising acatalytic converter, said catalytic converter comprising: a. asingle-piece, seamless metallic housing; b. a tubulated gas inlet portin said housing through which exhaust gas is introduced; c. a tubulatedgas outlet port in said housing through which the exhaust gas isdischarged; d. a tubulated intermediate section of said housing havingan inlet end and an outlet end; e. an inlet transition sectionconnecting said inlet port and said inlet end of said intermediatesection; f. an outlet transition section connecting said outlet end ofsaid intermediate section and said outlet port; and g. a plurality ofcascaded catalytic elements contained within said intermediate sectionand through which said exhaust gas passes sequentially when flowingbetween said gas inlet port and said gas outlet port.
 18. A multi-stagecatalytic converter for an internal combustion engine exhaust system,comprising: a. a single-piece, seamless metallic housing; b. a tubulatedgas inlet port in said housing through which exhaust gas is introduced;c. a tubulated gas outlet port in said housing through which the exhaustgas is discharged; d. a tubulated intermediate section of said housingconnecting said gas inlet port and said gas outlet port; e. an inlettransition section connecting said inlet port and said inlet end of saidintermediate section and comprising a plurality of cascaded subsections;f. an outlet transition section connecting said outlet end of saidintermediate section and said outlet port; and g. a plurality ofcascaded catalytic elements contained within said intermediate sectionand through which said exhaust gas passes sequentially when flowingbetween said gas inlet port and said gas outlet port; said catalyticconverter having been produced by a process comprising: i. providing aseamless metallic tube having a first end and a second end, said preformbeing adapted to be formed into said housing; ii. swaging said first endto form said gas inlet port and said inlet transition section; iii.swaging said second end to form said gas outlet port and said outlettransition section; iv. swaging said intermediate section to form saidsubsections; and v. inserting said catalytic elements in saidsubsections.
 19. A method for producing a catalytic converter for aninternal combustion engine exhaust system, said catalytic convertercomprising: a. a single-piece, seamless metallic housing; b. a tubulatedgas inlet port in said housing through which exhaust gas is introduced;c. a tubulated gas outlet port in said housing through which saidexhaust gas is discharged; d. a tubulated intermediate section of saidhousing connecting said gas inlet port and said gas outlet port; e. aninlet transition section connecting said inlet port and said inlet endof said intermediate section; f. an outlet transition section connectingsaid outlet end of said intermediate section and said outlet port; andg. a plurality of cascaded catalytic elements contained within saidintermediate section and through which said exhaust gas passessequentially when flowing between said gas inlet port and said gasoutlet port; and said method comprises: i. providing a seamless metallictube preform having a first end and a second end, said preform beingadapted to be formed into said housing; ii. inserting said plurality ofcascaded catalytic elements in said tube preform; iii. swaging saidfirst end to form said gas inlet port and said inlet transition section;and iv. swaging said second end to form said gas outlet port and saidoutlet transition section.
 20. A method as recited by claim 19, whereinsaid swagings of said first and second ends of said tube preform iscarried out in a plurality of graduated swaging steps.
 21. A method asrecited by claim 19, wherein said tube is gripped during said swaging ofsaid first and second ends.
 22. A method as recited by claim 19, whereineach of said catalytic elements comprises: a. a ceramic substrate havinga plurality of passages extending therethrough; and b. a catalyticallyactive material present on a substantial portion of the surface of eachof said passages; and c. an intumescent mat adapted to encircle saidceramic substrate and to seal the external surface of said ceramicsubstrate to the inner surface of said intermediate section; and saidmethod further comprises the step of: encircling each of said ceramicsubstrates with said intumescent mat to form said catalytic elementsprior to their insertion into said tube preform.
 23. A method as recitedby claim 19, wherein: a. said tube preform is provided from a supplytube; b. said swaging of said first end is carried out after saidcatalytic elements are inserted in said tube preform and said tubepreform is gripped during said swaging of said first end; c. said supplytube is cut to remove a portion thereof having a preselected lengthafter said first end is swaged, thereby defining said preform, and saidcatalytic elements being present in said preform; and d. said swaging ofsaid second end is carried out after said cutting of said supply tube toform said preform and said tube is gripped during said swaging of saidsecond end.
 24. A method as recited by claim 19, wherein said swagingsof said first and second ends are carried out simultaneously.
 25. Amethod for producing a multi-stage catalytic converter for an internalcombustion engine exhaust system, said catalytic converter comprising:a. a single-piece, seamless metallic housing; b. a tubulated gas inletport in said housing through which exhaust gas is introduced; c. atubulated gas outlet port in said through which the exhaust gas isdischarged; d. a tubulated intermediate section of said housingconnecting said gas inlet port and said gas outlet port, saidintermediate section comprising a plurality of cascaded subsections; e.an inlet transition section connecting said inlet port and said inletend of said intermediate section; f. an outlet transition sectionconnecting said outlet end of said intermediate section and said outletport; and g. a plurality of cascaded catalytic elements, each of saidelements being contained within one of said subsections, said exhaustgas passing sequentially through said elements when flowing between saidgas inlet port and said gas outlet port; and said method comprises: i.providing a seamless metallic tube preform having a first end and asecond end, said preform being adapted to be formed into said housing;ii. swaging said first end to form said gas inlet port and said inlettransition section; iii. swaging said second end to form said gas outletport and said outlet transition section; iv. swaging said intermediatesection to form said subsections; and v. inserting said plural cascadedcatalytic elements in said subsections.
 26. A method as recited by claim25, wherein said subsections have different diameters.
 27. A method asrecited by claim 26, wherein said subsections have sequentiallydecreasing diameters.
 28. A method as recited by claim 27, furthercomprising gripping said preform during at least a portion of saidswaging of said intermediate section, wherein said preform is engaged bygripping dies adapted to create a reduced diameter section in a portionof at least one of said subsections.
 29. A method as recited by claim25, wherein said gas inlet port, said gas outlet port, and saidsubsections are substantially coaxial.
 30. A method as recited by claim25, wherein said catalytic converter has at least one bend.
 31. A methodas recited by claim 25, wherein each of said catalytic elementscomprises: a. a ceramic substrate having a plurality of passagesextending therethrough; and b. a catalytically active material presenton a substantial portion of the surface of each of said passages; and c.an intumescent mat adapted to encircle said ceramic substrate and toseal the external surface of said ceramic substrate to the inner surfaceof said intermediate section; and said method further comprises the stepof encircling each of said ceramic substrates with said intumescent matto form said catalytic elements prior to their insertion into said tubepreform.